Method of washing caverns in salt formation



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6 INVENTORY I W/LLIAM R BAA/K5 F Arrow/Er United States Patent 3,326,666 METHOD OF WASHING (TAVERNS IN SALT FORMATION William P. Banks, Ponca City, Glrla, assignor to Continental Oil Company, Ponca City, Ulrla, a corporation of Delaware Filed Mar. 16, 1965, Ser. No. 440,250 16 Claims. (Cl. 299-) This invention relates to solution mining. In one specific aspect, this invention relates to a method for improving contact between the body of washing solution and the cavern surfaces.

In recent years solution mining of soluble salts has become of increasing importance. Such mining includes mining of sodium chloride, potassium chloride, sodium sulfate, trona and the like. Solution mining is also important in preparing underground caverns for storage of various fluids such as natural gas, ammonia, liquefied petroleum gas products, hydrocarbons of various types and the like.

In preparing such caverns, either for storage or solution mining, one or preferably two or more wells are drilled into formation and the formation fractured by well known fracturing methods. A solvent for the salt to be recovered is then pumped into the formation and the solvent enriched by the salt is recovered. When two or more wells are utilized, the solvent is pumped into one well and the solution recovered from the other well or wells. A problem encountered in such operations is that after the cavern reaches a certain size, the solvent adjacent to the Walls of the cavern is saturated with salt and the unsaturated solvent does not come into contact with the walls thereby greatly reducing the rate of salt recovery. When two or more wells are involved, the unsaturated solution tends to channel in the interior portion of the enlarged cavern thus bypassing most of the body of salt to be recovered. Sever-al methods have been suggested to improve contact. For example, in the single well operation, the solvent level in the well is kept low and the solvent sprayed radially from the well to the walls of the cavern. This method hastwo disadvantages, namely, the upper portion of the well only is reached by the fresh solvent, and the spray can reach outwardly only so far. In the multiple well systems, reversal of flow is often employed; however, even here channeling is'encountered. Cavern roof and cavern floors have been protected against further attack by either protecting the roof or floor by a non-solvent; however, this does not promote contact of that part of the unprotected cavern with unsaturated solvent.

. It is an object of this invention to provide a method of promoting contact of soluble cavern walls with unsaturated solvent thereby improving the efiiciency of the solvent utilized in removing the desired salt. Another object of this invention is to make it possible to etficiently wash a cavern of desired practical large size in a soluble formation using only a single well. Still other objects, features and advantages will be obvious to those skilled in the art from this specification and the claims.

The above objects are obtained according to this invention by providing a bubble-forming agent in the cavern. In one aspect, the bubble-forming agent is provided by incorporating into the solvent a gas under pressure. In another aspect of the invention the bubble-forming agent is supplied by introducing a readily vaporizable liquid, preferably having a specific gravity greater than the saturated solvent, into the cavern.

Since water is the most common solvent for solution mining, this invention will be described in terms of water solution mining of salt (NaCl). However, it should be understood that the method herein described is equally applicable to other solvents and other materials soluble in the solvent.

The invention involves the use of injection solvent containing dissolved gas or a liquefied gas preferably having a specific gravity greater than the solvent. The solvent and dissolved gas or liquefied gas are injected into the cavity so as to fill the void space therein. Then these two fluids are produced at one or more wells so that the pressure in the formation drops sufliciently for the dissolved gas or liquefied gas to vaporize and form gas bubbles. These gas bubbles rise upward through the solution and thereby stir the body of the solution in the salt formation void space. This stirring of the solution promotes contact between the unsaturated solution and the walls of the soluble formation thereby increasing the rate of solution of the soluble material in the liquid. If desired, a surface active agent can be added to the solvent to facilitate the formation of gas bubbles in the solution when the pres-v sure is reduced.

It is particularly advantageous to use the liquefied gases which are more dense than the solvent and would therefore sink to the bottom of the cavern. As the pressure is reduced on the solution, the liquefied gas forms a steady stream of gas bubbles that stir the solution over a prolonged time. It is within the scope of the invention to recover the gas from the solution production and recycle the gas.

In some cases, a liquefied gas having a specific gravity less than that of the solvent can be utilized. This would be true, for example, if the injection well roof is below the recovery well. The light Weight liquefied gas would collect above the solvent and as the pressure is reduced the liquefied gas would begin to vaporize pushing the liquid down into the solvent and thereby forming gas bubbles or the light weight liquid could collect in a pocket along the cavern ceiling and react in a similar manner.

The solvent will depend largely upon the nature of the soluble material to be recovered. These are generally water or brine.

Suitable gases which are soluble include CO propane and similar light weight hydrocarbons. Suitable heavy liquefied gases include S0 CCI F sold as Freon 12', CCIF H sold as Freon 22 and the like. Suitable light weight liquefied gases include liquid ethane, propane, and the like.

To further illustrate the invention, reference is made to the attached drawings of which:

FIGURE 1 is a schematic drawing of a cavern containing a single well above; and

FIGURE 2 is a schematic drawing of a cavern containing two well bores.

Referring now to the drawings, in FIGURE 1, cavern 1 in salt formation 2 is penetrated by well 3. This well is comprised of casing 4 and inner conduit 5. The casing 4 is cemented 7 in the overburden, rock formation 6. In the operation of the system, water is passed via conduit 8 and pump 9 to mixing T 10. At the same time, liquefied gas is passed via conduit 11 and compressor 12 to mixing T 10. The mixture then passes under pressure via conduit 5 to the cavern 1 where the liquefied gas settles to the bottom of the cavern forming layer 20. The back pressure on cavern 1 is maintained by back pressure regulator 14 so as to control the release ofgas 21 at the desired rate. The gas being released stirs the liquid in cavern 1 thereby causing greater contact between the liquid and the cavern walls. The resulting solution is removed via the annulus formed between casing 4 and conduit 5 to conduit 13 and then passes through back pressure regulator 14 to salt and/or gas recovery system, not shown. It is, of course, within the knowledge of the art that the fresh solvent can be passed downward via the annulus and the solution recovered via conduit 5'. In either case, it is advantageous to provide means for lowering the injection tube as the cavern deepens and, therefore, the particular arrangement shown is preferred. Where a liquefied gas is utilized in lieu of a soluble gas, it is not necessary that the solvent and gas be passed simultaneously to the cavern, since the heavy liquefied gas will settle to the bottom or the solvent will float. It is also obvious that the gas and solvent can be premixed under pressure and introduced to the cavern in which the case the T It} will be simply an elbow fitting.

Referring now to FIGURE 2, the cavern 1 in salt bed 2 is penetrated by not only Well 3 but also Well 16. The reference numerals 114 and -21 are the same as in FIGURE 1. The well bore 16 comprises casing 17 and conduit 18. In the two-well system, solvent and liquefied gas passes down conduit 5 to cavern 1 and is withdrawn via conduit 18 to back pressure control valve 14 and then to recovery via conduit 19. From time to time, the flow can be reversed. As in FIGURE 1, the conduit 5 and conduit 18 are vertically adjustable.

To further illustrate the invention, two specific examples will be given. For simplicity of calculations it is assumed that overburden pressure on the cavern is one pound per square inch per foot of depth, and hydrostatic pressure is one-half pound per square inch per foot of depth.

Example 1 A cavern having 600 feet of overburden, i.e., distance from cavern roof to earths surface, will support 600 pounds pressure without lifting the overburden and therefore 600 p.s.i.a. is the maximum pressure which can be applied. On the other hand, the hydrostatic head on the well will be 300 p.s.i.a. Now, if CO is compressed to something under 600 p.s.i.a, say 550 p.s.i.a via compressor 12 and is injected with water via pump 9 and conduit 5 at 550 p.s.i.a at a formation temperature of 80 F., the solubility of CO in the water will be about 3.8 pounds CO per 100 pounds of water. The mixture is pumped into the well until CO appears at the production well, e.g. conduit 11 whether from the single bore or multiple bore system. At this point, the pumping of water and CO is interrupted and the pressure gradually lowered via back pressure regulator 14. Since the pressure in the cavern exceeds the hydrostatic head in the well, fluid will be produced and the pressure gradually lowered. As the pressure is lowered the solubility of the CO in water is also lowered and gas bubbles are formed in the solution in the cavern causing the solution to be stirred. As an alternate method of operation, the water and CO can be continuously passed to the cavern and solution continuously removed. The difference in pressure maintained by the back pressure regulator 14 will control the rate of production and the rate of gas bubble formation. In either case, the bubble will form and stir the solution in the cavern. The solubility of CO in the water at 80 F. and 300 p.s.i.a. is 2.3 pounds of CO per hundred pounds of water. Thus, each cubic foot of water (62.4 lbs.) will release about 0.95 lb. CO when the pressure is lowered from 550 p.s.i.a. to 300 p.s.i.a. This then yields about 0.3 cu. ft. of gas per cu. ft. of water which is available for agitating the solution, thus promoting contact of the cavern walls with unsaturated solution.

Example 2 In this case the cavern is covered with 80 ft. of overburden thus limiting the maximum pressure to about 80 p.s.i.a. Again, assuming 80 F. formation temperature water and S0 are pumped into the cavern at 70 p.s.i.a, the S0 will be liquid and has a specific gravity of about 1.4. The solution specific gravity will be something over 1 but less than 1.4. Therefore, under the injection conditions, the liquid S0 will sink to the bottom of the cavity. However, since the hydrostatic head is only about 40 p.s.i.a, if we allow the pressure to drop below 59 p.s.i.a. (the vapor pressure of liquefied S0 the liquefied S0 will vaporize and agitate the liquid in the cavern. As in the above Example 1, we can operate intermittently or continuously by regulating the back pressure.

It will be understood by those skilled in the art that the drawing and examples are illustrative only, and modifications are Within the skill of the art, having been given the broad concept.

Having thus described the invention, I claim:

1. A method for improving contact between soluble salt cavern walls with salt unsaturated solvent which comprises introducing into said cavern a solvent and a nongaseous bubble forming material under conditions wherein said bubble forming material releases gas bubbles in said solvent under controlled conditions and at a controlled rate thereby agitating said solvent.

2. The method of claim 1 wherein said bubble forming material is a liquid having a specific gravity less than said solvent.

3. The method of claim 1 wherein said bubble forming material is a liquid having a specific gravity greater than said solvent.

4. The method of claim 3 wherein said solvent is water.

5. The method of claim 4 wherein said liquid is S0 6. The method of claim 4 wherein said liquid is CCl F 7. The method of claim 4 wherein said liquid is CClF H 8. The method for agitating a solvent in a cavern formed in a soluble bed said method comprising introducing into said cavern solvent and a gas forming compound under sufficient pressure to prevent said gas forming compound from forming substantial amounts of gas, lowering the pressure on said cavern until the said gas forming compound forms relatively large amounts of gas thereby producing bubbles in said solvent and agitating same.

9. The method of claim 8 wherein a surfactant is introduced into said cavern.

10. The method of claim 8 wherein said gas forming compound is a liquid at the injection pressure and has a specific gravity greater than said solvent and the pressure on said cavern is lowered to at least the vaporization pressure of said liquid.

11. The method of claim 10 wherein said soluble bed is water soluble and said solvent is water.

12. The method of claim 11 wherein said liquid is S0 13. The method of claim 11 wherein said liquid is CCl F 14. The method of claim 11 wherein said liquid is CCIF H.

15. The method of claim 8 wherein gas forming compound is comprised of at least two gas forming compounds.

16. The method of claim 15 wherein one of said gas forming compounds is an immiscible liquid.

References Cited UNITED STATES PATENTS 2,822,158 2/1958 Brinton 2994 2,981,071 4/1961 Brandt 61.5 3,130,960 4/1964 Martin 299-5 3,284,137 11/1966 Wolber 299-5 ERNEST R. PURSER, Primary Examiner. 

1. A METHOD FOR IMPROVING CONTACT BETWEEN SOLUBLE SALT CAVERN WALLS WITH SALT UNSATURATED SOLVENT WHICH COMPRISES INTRODUCING INTO SAID CAVERN A SOLVENT AND A NONGASEOUS BUBBLE FORMING MATERIAL UNDER CONDITIONS WHERE- 